When a vessel revolves about an external axis and also rotates about its internal axis, the contents of the vessel, typically abrasive pieces and objects to be finished by contact with the abrasive pieces, experience centrifugal and rotational forces. The centrifugal force relates to the pressure experienced by the contents, while the rotational force relates to the speed of the contents relative to each other. Various types of finishes can be achieved due to the presence of the centrifugal and rotational forces. Furthermore, centrifugal finishers having several separate inner vessels within an outer vessel are useful when objects being finished need to be kept separated from each other for identification or functional reasons, or when different types of abrasive pieces are used simultaneously on separate objects.
Machines for subjecting objects to both centrifugal and rotational forces are well known. One type of machine relies strictly on a complicated set of gears, such as planetary gears, and is limited to a particular ratio of revolutional speed to rotational speed. This apparatus is complicated since it requires many parts and is quite noisy. Also, the number of vessels is fundamentally constrained for a certain gear ratio, since as the number of vessels increases (the footprint of each vessel decreases), the gear size must also increase until the gears bump into each other. If the gear ratio is increased, finer gears are required, and the strength of the gear material becomes a constraint.
Another type of machine creates centrifugal force by revolving a vessel around a shaft and creates rotational force using a belt wrapped around the shaft and the exterior of the vessel. Here, the speed of the belt is related to the speed of the shaft. Furthermore, mechanical failures occur due to the high stress sustained by the belt and/or parts restraining the vessel. Overheating is also common in these machines.
None of these prior art machines permits separate control, over a wide range, of the rotational and centrifugal forces experienced by the contents of a vessel using an apparatus of simple construction which operates quietly and in which the parts are not subjected to high stress.